The invention relates to an integrated circuit comprising a transistor, a phase-shifting arrangement which produces a phase shift which is controllable by applying a control signal to a control input thereof, which arrangement is included in a signal path which extends through said transistor, and a control signal generator circuit the output of which is coupled to said control input, said control signal generator circuit including a circuit element comprising doped zones which correspond to the collector, base and emitter zones of the transistor respectively, said element being included in the control signal generator circuit in such manner that the value of the output signal of said generator circuit will depend on the value of an electrical property of said element.
One circuit of this type is known from U.S. Pat. No. 3,597,639. In this known circuit the said circuit, element is a diode-connected second transistor, the commoned collector and base of which are connected to a constant-potential point via a variable resistor, and the emitter of which is connected to the emitter of a third transistor included in the phase-shifting arrangement. The function of the series combination of the variable resistor and the second transistor is to control the current flowing through the third transistor in accordance with the setting of the variable resistor, this current determining the phase shift produced by the phase-shifting arrangement. The function of the second transistor is to provide temperature tracking and biasing stabilization.
It is known that the transition frequency f.sub.T of a transistor is related to the width of the base region of the transistor; the smaller the width the higher is the value of f.sub.T. When a signal is transmitted through a signal path which extends through the transistor the signal will be subjected to a phase lag in the transistor due to the finite value of the transistor transition frequency. If the frequency of the signal is far below the transition frequency this phase lag will normally be negligible. However, if the frequency of the signal is approaching the transition frequency this will not be the case, and the phase lag produced may have to be taken into account in the design of the circuit including the transistor. For example, if the transistor forms part of a gyrator circuit arrangement one port of which is loaded by a capacitance so that the other port is inductive, this other port being connected to a capacitance to form a parallel or series resonant circuit, the quality factor Q of the resonant circuit will be enhanced if the phase lag produced by the transistor in signals passing through it is significant unless this phase lag is compensated for in some way. In such a case, if the resonant circuit is to have the quality factor required either its basic parameters must be chosen to give this quality factor and phase compensation must be provided, or its basic parameters must be chosen to give a quality factor which is just that amount lower than that which is required which will result in the required quality factor being obtained after its enhancement due to the phase lag.
Current manufacturing techniques for integrated circuits are such that the widths of the base regions, and hence the transition frequencies, of transistors included in the circuits may vary by a considerable amount from batch to batch. Therefore, if the aforementioned resonant circuit where the inductive element is formed by a capacitively-loaded gyrator is constructed as an integrated circuit, the phase lags occurring in the various transistors of the gyrator will be largely unpredictable, with the result that the quality factor of the resonant circuit will also be largely unpredictable if the resonant frequency of the circuit is such that these phase lags are significant. This is true even if purely fixed, i.e. base-width-independent, phase compensation is incorporated in the circuit. If the quality factor required has to be met with only a narrow allowable tolerance this may result in a large number of rejects occurring during mass production. It is an object of the invention to provide means whereby this disadvantage may be mitigated.